Zirconium-aluminum-nickel cathodes



Yield Strength PS].

March 14, 1961 BOUNDS ET AL 2,975,050

ZIRCONIUM-ALUMINUM-NICKEL CATHODES Filed June 4, 1957 4 Sheets-Sheet 1March 14, 1961 ET AL 2,975,050

ZIRCONIUM-ALUMINUM-NICKEL CATHODES 4 Sheets-Sheet 2 Filed June 4, 1957OmVN 3 929w BE (v n l March 14, 1961 A, BOUNDS ET AL 2,975,050

ZIRCONIUM-ALUMINUM-NICKEL CATHODES 4 Sheets-Sheet 3 Filed June 4, 1957 lwf 4.5)vs.Life

Hours of Life Hours of Life Hours of Life March 14, 1961 Filed June 4,1957 A. M. BOUNDS ET AL 5,050

ZIRCONIUM-ALUMINUM-NICKEL CATHODES 4 Sheets-Sheet 4 Fig. 4

l FM vs,Life

Hours of Life I FM vs. Life 4600 s'ooo 6000 Hours of Life lo'oo 26003000 barium, strontium or the like.

coating adversely affects the tube operation.

United. States PatentlfO ZIRCONIUM-ALUMENUM-NICKEL CATHODES Ardrey M.Bounds, Laverock, and Richard L. Hoif, Norristown, Pa., assignors toSuperior Tube Company, Norristown, Pa., a corporation of PennsylvaniaFiled June 4, 1957, $81. No. 663,525

4 Claims. cl. 75-170 This invention relates to electron tube cathodes ofthe indirectly heated type as distinguished from cathodes of thedirectly-heated filamentary type.

In general, the object of the invention is to provide indirectly-heatedcathodes which, as compared to prior nickel alloy cathodes, havesubstantially increased resistance to deformation at cathode-operatingtemperatures and which have emission and sublimation characteristicssubstantially equivalent to or better than those of the prior nickelalloy cathodes.

In accordance. with the present invention, such objective is attained bymaking the cathode sleeves, or the like, from nickel cathode alloyswhich include zirconium and aluminum within the low, narrow percentagelimits hereinafter specified.

In the following description, reference is made to the accompanyingdrawings, in which:

Fig. 1 is a group of curves exemplary of the yield strength, atcathode-operating temperatures, of several zirconium-aluminum-nickelalloys and of a reference nickel cathode alloy; and

Figs. 2, 2A, 3A, 3B, 3C, 4 and 4A each comprises groups of curvesreferred to in discussion of the emission characteristics ofindirectly-heated cathodes of the reference nickel alloy and ofzirconium-aluminum-nickel allow.

In general, indirectly-heated cathodes consists of a nickel alloy baseelement, such as a sleeve or cup, having thereon a coating of alkalineearth metals, such as The fabrication of the alloy stock into cathodebase elements involves hot and cold Working steps such as forging,rolling, drawing, stamping and the like. After assembly of the coatedcathode, including its heater, and other electrodes within an envelopeto form an electronic tube, the cathode is activated by temporarilyheating it substantially above its normal operating temperature. Duringactivation, there are reactions between the base element materials andthe coating materials which convert the coating to a combination ofcomplex oxides suited to emit electrons when heated to cathode-operatingtemperatures. In the more usual types of service, the life of a tube isconsidered terminated when its cathode emission is definitely subnormalat normal heater current. For many uses, including field service wherethe available heater supply voltage is low or fluctuating, tubes areconsidered unfit for use when the cathode emission is substantiallyaffected by low or varying heater temperature.

The-operating life of a tube is also afiected by cathode characteristicsother than electron emission. Eruptive flaking or peeling of the cathodecoating shortens .the

normal life of tubes, particularly high-voltage rectifier tubes. Theformation and growth of a high-impedance interface between the cathodebase element and its oxide The re sistive component of such interface isdamging, particularly in pulsed type service, even at ordinaryfrequencies: the capacitive component of such interface impedance isPatented Mar. 14, 9 1

particularly damaging at high frequencies-even when the tube is notoperated under pulsed or cut-off conditions. The operating life of atube may also be effectively terminated by the formation, from materialsublimed from the cathode, of a leakage path between electrodes of thetube.

In addition to' such electrical characteristics, the operating life of atube is also determined by the physical or mechanical characteristics ofits cathode. Cathode sleeves made of the usual nickel alloys often bowedwhen subjected to high activation temperatures, causing internalshort-circuits or changes in interelectrode spacing. Also in serviceswhere the tubes were subjected to severe mechanical shock, as inairborne missile equipment, buckling or deformation of their cathodesleeves rendered the tubes inoperative before performance of theirintended function.

We have determined that addition of zirconium and aluminum, within low,narrow percentage ranges later specified, to nickel cathode alloysprovide indirectlyheated cathodes, which at cathode-operatingtemperatures, have a hot strength from about two to over three andone-half times that of the nickel cathode alloy; which have, during thenormal life expectancy of nickel cathodes, a high level of stableemission substantially equivalent to or better than that of nickel alloycathodes; and some of which have high level emission for life ex-vtending substantially beyond that of nickel alloy cathodes.

Furthermore, such zirconium-aluminum-nickel alloys are amenable to hotand cold metal-working steps incident to fabrication of cathode sleevesand other indirectlyheated cathode elements from the alloy stock. Suchzirconium-aluminum-nickel alloy cathodes also exhibit thecharacteristics of virtual freedom from sublimation and of negligibleinterface impedance.

Considering firstthe enhancement of yield strength at cathode-operatingtemperatures of about 1650 F.,

bled by increasing the percentage of zirconium substantially beyond0.l%-up to about 0.5%. Within such range of addition of zirconium,aluminum may also be added, but not in excess of about 0.2%. Frommetallographic investigation, it appears that addition of zirconium andaluminum produces precipitation-hardening. However, the increasedstrength, due to such effect, falls off at temperatures of 1500 F. orhigher. The 0.5% upper limit specified for addition of zicronium shouldnot be greatly exceeded as there is danger of incipient melting duringthe exhaust schedules used to activate the cathodes. The upper limitspecified. for addition of aluminum should not be appreciably exceededbecause of flaking of the cathode coating and the re- I sultantimpairment of the bond which forms at the interface between the cathodesleeve and its emissive oxide coating.

Except for residuals, the balance of the new cathode alloys isessentially the nickel base usually including cobalt not in excess of1%. Higher percentages of. cobalt,

up to 10% or 15% in the nickel base, have .been found to have littleefiect upon the emission characteristics or upon the mechanicalstrengthat cathode-operating temperatures. I I

We have found that zirconium, besides being a very effectivestrengthening agent for nickel cathode alloys,

is also an efiective activating agent for such alloys. Consequently, theuse of magnesium and silicon as activating agents may be minimized oravoided. This is of advantage because magnesium is largely responsiblefor formation of sublimation deposits leading to interelectrode leakagepaths and because silicon is largely responsible for formation of a highinterface-impedance between cathode sleeves and their coatings- In thezirconiumaluminum-nickel alloys, both the zirconium and aluminum areactivating agents. Considering both the mechanical and emissioncharacteristics of indirectlyhcated cathodes, the preferred alloy'shouldcontain about 0.25% zirconium and 0.08% aluminum, balance nickel.

In determination of the limits of addition of zirconium and aluminum,both for obtaining enhanced hot-strength of indirectly-heated cathodesand for preservation or enhancement of their electrical properties,tests were conducted on a series of zirconium-nickel andzirconiumaluminum-nickel alloys: specific examples of such alloys Forcomparison purposes, the emission curves of concurrently tested tubeshaving #220 nickel cathodes are also shown in Figs. 2-4A.

Referring to Figs. 2-4A, the indirectly-heated cathodes of the #54016and #54018 alloys activated as rapidly or more rapidly than the #220nickel alloy cathodes, although containing much lower percentages of theactivating agents, magnesium and silicon, than the reference alloy.

After activation, the emission and FM characteristics of the #54016alloy cathodes remained high and stable, somewhat surpassing thosecharacteristics of the #220 nickel cathodes for the usual lifeexpectancy of 1000 hours (Figs. 3, 3A, 4). During an extended 6500 hourlife test (Figs. 2A, 3B, 4A), the emission characteristics of the #54016cathodes remained relatively high and stablesubstantially surpassingthose of the #220 cathodes at all readings beyond 2000 hours of life.The #54016 cathodes exhibited good coating adherence durare listed belowin Table A. ing the 6500-hour life test, whereas during that extendedTable A Alloy Zr Al Mg Si Fe Mn 0 Cu Co Ni #54016 .137 .005 .009 .005.02 .06 .05 .009 .071 Essentially #54018 .11 .11 .608 .000 .079 .08 .038.008 .05 Remain #5525 .43 .12 .025 .006 .017 .05 .042 .004 .62 der.

#220 0 .009 .025 .014 .032 .05 .04 .014 .103 Essentially (Iitemain- Asshown by the test. curves of Fig. 1, the yield strengths of thezirconium-bearing alloys of Table A are significantly higher than thatof the reference nickel cathode alloy throughout a rangeof hightemperatures including the usual cathode-operating temperatures of about1600 F. In general, as determined by these and other tests, the hotyield strength of the zirconium-aluminum-nickel alloys was at leasttwice that of the nickel cathode alloy #220; the hot yield strength ofalloy #5525 was nearly four times that of reference alloy #220 at 1650F. As confirmed by shock tests on tubes with cathodes at operatingtemperatures, there is direct correlation between the shock deformationcharacteristics of indirectly-heated cathodes and the hot yield strengthof the cathode alloy.

The emission characteristics of oxide-coated indirectlyheated cathodesusing the alloys of Table A for the cathode sleeves are shown in Figs.2-4A. In these figures, the curves are identified by the respectivealloy designations of Table A. For these emission tests, the cathodeswere incorporated in the standard diode structure defined in Spec.F-270-52T of ASTM (American Society of Testing Materials). The cathodesleeves were 0.045 OD. x 0.002" wall it 27 mm. long. The life burningconditions for the emission tests were: an anode-cathode supply voltage(E of 100 volts; a heater voltage (E) of 6.5 volts, and a loadresistance (R of 1000 ohms.

Anode current readings were taken at 0, 5, 25, 50, 100, 200, 350 and 500hours, and then every 250 hours to the end of the test. At each testperiod, the anode current was read at a plate voltage of volts for aseries of heater voltages, including the normal voltage (E v.) andsubnormalvoltage (including E =4.5 v.). Such anode current readingsplotted against time constitute the curves of Figs. 2-3C. The I FM ordirect-current Figure of Merit curves of Figs. 4, 4A are derived fromthe anode current vs. heater'voltage readings as described in detail inan article of Briggs and Richard in the ASTM Bulletin of January 1951.Briefly, the I FM value is the ratio. of the I ,E coordinates at theknee of the anode current/heater voltage curve where the anode-currentchanges from a space charge limited condition to a temperaturerlimitedcondition (subnormal heater voltage).

time the coating adherence of the #220 nickel cathodes suffered severedepreciation. For the #54016 cathodes, no sublimation deposit could bedetected until after 2700 hours. Even after 6500 hours, only a fainttrace was discernible, whereas for the #220 nickel cathodes asublimation deposit was visible at 25 hours, and became a heavy depositat the end of the 6500-hour test. The interface-impedance of the #54016cathodes remained negligible throughout the extended life test.

After activation, the emission and FM characteristics of the #54018alloy cathodes remained high and stable and were closely equivalent toor better than those of the #220 nickel cathodes for the usual lifeexpectancy of 1000 hours (Figs. 2, 3C, 4). The #54018 alloy cathodesexhibited good coating adherence and negligible interfaceimpedancethroughout the 1000-hour life test period. For the #54018 cathodes, nosublimation deposit was visible until after 500 hours of life and therestill was only a trace at 1000 hours of life. For the #220 nickelcathodes, a sublimation deposit was visible at 25 hours and was quiteheavy at 1000 hours.

Although the #5525 alloy activated rapidly to a favorably high emissionlevel, the emission dropped quickly due to coating peel. Such coatingpeel was attributed to interaction between the aluminum and zirconiumwhich in this alloy were respectively 0.12% aluminum and 0.43%zirconium. Such interaction was also indicated by the lower amenabilityto cold drawing characteristics of this higher aluminum alloy. It wasconcluded that the proportion of aluminum should be reduced as thezirconium content is increased to obtain long cathode life and to avoidfabrication difiiculties.

In brief rsum, the zirconium-aluminum-nickel cathodes have a resistanceto deformation which is from about two to four times greater than thatof the reference nickel cathodes; their emission characteristics areequivalent to or better than those of the reference nickel cathodes forthe normal life expectancy of the latter and continue to be at highlevel substantially beyond the normal life expectancy of the referencenickel cathodes; and their sublimation characteristics are substantiallybetter than those of the reference nickel cathodes.

What is claimed is:

1. A cathode structure of the indirectly-heated nonfilarnentary typeincluding sleeves and cups characterized by high strength atcathode-operating temperatures, rapid activation, and good emission andsublimation characteristics and composed of an alloy tree of tungstenand containing zirconium in the range of 0.05% to 0.5% by weight,aluminum 0.005% to 0.2% by weight, and the remainder essentially nickel.

2. An indirectly-heated cathode structure as in claim 1 in which thealloy compositionby weight is:

Percent Zirconium 0.05 to 0.5 Aluminum 0.005 to 0.2 Magnesium not morethan 0.07 Silicon not more than 0.05 Carbon not more than 0.08 Manganesenot more than 0.15 Cooper not more than 0.05

and the remainder essentially nickel.

3. A cathode structure of the indirectly-heated nonfilamentary typeincluding sleeves and cups in which the composition by weight is about0.25% zirconium, 0.08% aluminum, balance substantially a tungsten freenickel base.

minum being in the range of 0.005% to 0.2% by weight, the maximumpercentage of aluminum for a given percentage of zircomium varying from0.2% for the lower limit of zirconium to 0.005% for the upper limit ofzitconium.

References Cited in the file of this patent UNITED STATES PATENTS 4. Acathode structure of the indirectly-heated nonfilamentary type includingsleeves and cups composed of a nickel-base alloy which is free oftungsten and which, contains zirconium and aluminum, the zirconium beingin the range of 0.05% to 0.5% by weight and the al-u-

1. A CATHODE STRUCTURE OF THE INDIRECTLY-HEATED NONFILAMENTARY TYPEINCLUDING SLEEVES AND CUPS CHARACTERIZED BY HIGH STRENGTH ATCATHODE-OPERATING TEMPERATURES, RAPID ACTIVATION, AND GOOD EMISSION ANDSUBLIMATION CHARACTERISTICS AND COMPOSED OF AN ALLOY FREE OF TUNGSTENAND CONTAINING ZIRCONIUM IN THE RANGE OF 0.05% TO 0.5%